Low perfusion and missed diagnosis of hypoxemia by pulse oximetry in darkly pigmented skin: A prospective study

Importance Retrospective clinical trials of pulse oximeter accuracy report more frequent missed diagnoses of hypoxemia in hospitalized Black patients than White patients, differences that may contribute to racial disparities in health and health care. Retrospective studies have limitations including mistiming of blood samples and oximeter readings, inconsistent use of functional versus fractional saturation, and self-reported race used as a surrogate for skin color. Understanding the cause of biased readings by pulse oximetry in patients with darkly pigmented skin is high priority given the essential nature of pulse oximetry. Objective To prospectively measure the contributions of skin pigmentation, perfusion index, sex, and age on pulse oximeter errors. Design We studied two pulse oximeters (Nellcor N-595™ and Masimo Radical 7™) in prevalent use in North America, Europe, and Asia-Pacific regions. We analyzed 9,763 matched pulse oximeter readings (SpO2) and arterial oxygen saturation (hemoximetry SaO2) during stable hypoxemia (SaO2 68-100%). Perfusion index (PI) was measured as percent infrared light modulation by the pulse detected by the pulse oximeter probe, with low perfusion categorized as PI <1%. Setting Clinical research laboratory Participants 146 healthy subjects, including 25 with light skin (Fitzpatrick class I-II), 78 with medium (class III-IV), and 43 with dark (class V-VI) skin. Exposures Controlled hypoxemia Main Outcomes Pulse oximeter bias (difference between SaO2 and SpO2) by skin pigment category in a multivariable mixed-effects model incorporating repeated-measures and different levels of SaO2 and perfusion. Results Skin pigment, perfusion index and degree of hypoxemia significantly contributed to errors (bias) in both pulse oximeters. The combined frequency of missed diagnosis of hypoxemia (pulse oximeter readings 92-96% when arterial oxygen saturation was <88%) in low perfusion conditions was 1.1% for light, 8.2% for medium and 21.1% for dark skin. Conclusions and Relevance Low peripheral perfusion combined with darker skin pigmentation leads to clinically significant high-reading pulse oximeter errors and missed diagnoses of hypoxemia. Darkly pigmented skin and low perfusion states are likely the cause of racial differences in pulse oximeter performance in retrospective studies. Both skin pigmentation and low perfusion should be accounted for in regulatory standards for pulse oximeters. Key Points Question Laboratory validation of pulse oximeter performance has found errors in Black subjects that are smaller than those from recent reports of hospitalized Black patients. We test the hypothesis that low perfusion amplifies pulse oximeter error in subjects with dark skin during hypoxemia. Findings Dark skin pigmentation combined with low perfusion produces large errors in pulse oximeter readings in healthy subjects during hypoxemia, sufficient to miss a diagnosis of hypoxemia in about 20% of readings. Meaning Accuracy of pulse oximeters in the diagnosis of hypoxemia is impaired by a combination of low perfusion and dark skin pigmentation. Low perfusion should be accounted for in future testing and regulatory guidelines for pulse oximeters to improve performance and minimize bias in patients with dark skin pigment.